Man-made binding proteins, in particular monoclonal antibodies and engineered antibodies or antibody fragments, have been tested widely and shown to be of value in detection and treatment of various human disorders, including cancers, autoimmune diseases, infectious diseases, inflammatory diseases, and cardiovascular diseases (Filpula and McGuire, Exp. Opin. Ther. Patents (1999) 9: 231-245). For example, antibodies labeled with radioactive isotopes have been tested to visualize tumors after injection to a patient using detectors available in the art. The clinical utility of an antibody or an antibody-derived agent is primarily dependent on its ability to bind to a specific targeted antigen. Selectivity is valuable for delivering a diagnostic or therapeutic agent, such as isotopes, drugs, toxins, cytokines, hormones, growth factors, enzymes, conjugates, radionuclides, or metals, to a target location during the detection and treatment phases of a human disorder, particularly if the diagnostic or therapeutic agent is toxic to normal tissue in the body.
The potential limitations of antibody systems are discussed in Goldenberg, The American Journal of Medicine (1993) 94: 298-299. The important parameters in the detection and treatment techniques are the amount of the injected dose specifically localized at the site(s) where target cells are present and the uptake ratio, i.e. the ratio of (the concentration of specifically bound antibody to that of the radioactivity present in surrounding normal tissues. When an antibody is injected into the blood stream, it passes through a number of compartments as it is metabolized and excreted. The antibody must be able to locate and bind to the target cell antigen while passing through the rest of the body. Factors that control antigen targeting include location, size, antigen density, antigen accessibility, cellular composition of pathologic tissue, and the pharmacokinetics of the targeting antibodies. Other factors that specifically affect tumor targeting by antibodies include expression of the target antigens, both in tumor and other tissues, and bone marrow toxicity resulting from the slow blood-clearance of the radiolabeled antibodies. The amount of targeting antibodies accreted by the targeted tumor cells is influenced by the vascularization and barriers to antibody penetration of tumors, as well as intratumoral pressure. Non-specific uptake by non-target organs such as the liver, kidneys or hone-marrow is another potential limitation of the technique, especially for radioimmunotherapy, where irradiation of the bone marrow often causes the dose-limiting toxicity.
One suggested approach, referred to as direct targeting, is a technique designed to target specific antigens with antibodies carrying diagnostic or therapeutic radioisotopes. In the context of tumors, the direct targeting approach utilizes a radiolabeled anti-tumor monospecific antibody that recognizes the target tumor through its antigens. The technique involves injecting the labeled monospecific antibody into the patient and allowing the antibody to localize at the target tumor to obtain diagnostic or therapeutic benefits. The unbound antibody clears the body. This approach can be used to diagnose or treat additional mammalian disorders.
Another suggested solution, referred to as the “Affinity Enhancement System” (AES), is a technique especially designed to overcome deficiencies of tumor targeting by antibodies carrying diagnostic or therapeutic radioisotopes (U.S. Pat. No. 5,256,395 (1993), Barbet et al., Cancer Biotherapy & Radiopharmaceuticals (1999) 14: 153-166). The AES utilizes a radiolabeled hapten and an anti-tumor/anti-hapten bispecific binding protein that recognizes both the target tumor and the radioactive hapten. Haptens with higher valency and binding proteins with higher specificity may also be utilized for this procedure. The technique involves injecting the binding protein into the patient and allowing it to localize at the target tumor. After a sufficient amount of time for the unbound binding protein to clear from the blood stream, the radiolabeled hapten is administered. The hapten binds to the antibody-antigen complex located at the site of the target cell to obtain diagnostic or therapeutic benefits. The unbound hapten clears the body. Barbet mentions the possibility that a bivalent hapten may crosslink with a bispecific antibody, when the latter is bound to the tumor surface. As a result, the radiolabeled complex is more stable and stays at the tumor for a longer period of time. This system can be used to diagnose or treat mammalian disorders.
There remains a need in the art for production of multivalent, monospecific binding proteins that are useful in a direct targeting system and for production of multivalent, multispecific binding proteins that are useful in an affinity enhancement system. Specifically, there remains a need for a binding protein that exhibits enhanced uptake at targeted antigens, decreased concentration in the blood, and optimal protection of normal tissues and cells from toxic pharmaceuticals.